Computer peripherals as for example floppy disk drive mechanisms, fixed memory devices, magnetic disks etc. are usually connected to a computer system or computer systems via a device interface. Most of the time this device interface is controlled by separate interface controls which are connected, via a computer bus system with a central processing unit, generally called CPU of a computer system and in the peripheral installations with the central control thereof. Via initialization of the central processing installation, data can be transmitted from there to the computer peripherals, and e.g. stored there or, data stored there can be forwarded to the central processing apparatus for further processing.
For the connection of the most various peripheral memory and output installations to small computer systems, a standardized device interface has been employed for quite some time; the "Small Computer System Interface", established in the document X3T9.2 of ANSI (American Nationals Standard Institute). To this device interface several computer peripherals can be connected in parallel fashion The standardized device interface has 50 connecting points whereby maximally 25 interface connections, i.e. two connecting points per interface connection, can be realized. For the bidirectional transmission of addresses and data, the device interface is equipped with 8 data interface contacts and with a data parity bit interface contact. For the control of the device interface, a further nine control contacts are provided. These are, respectively:
a bidirectionally controllable "busy" interface contact, where it is indicated either from a computer peripheral or a computer system that the device interface has already been switched active. Such an actively switched device interface must not be commanded regarding data or address transmission by any other computer peripheral or a computer system.
a bidirectionally controllable "Select" interface contact, via which together with the feeding of target adresses at the data interface contacts from a computer system or from a computer peripheral, a target computer system/predictor or a target computer peripheral is selected.
a data/control interface contact acting from the computer peripheral to the computer systems at which the current transmission of data or control information via the data interface contacts is indicated.
an input/output interface contact acting from the computer peripheral to the computer systems, with which the direction of the data transmitted via the data interface contacts is controlled.
a communication interface contact effective from the computer peripheral to the computer systems, at which the current transmission of data via the data interface contacts is indicated.
an "acknowledge" interface contact effective from the computer systems to the computer peripheral, at which the acknowledgement information on the data interface contacts during an inquiry/acknowledge transmission operation is indicated.
a "request" contact effective from the computer peripheral to the computer systems, at which the request information on the data interface contacts during a request/acknowledge-transmission operation is indicated.
an "attention" interface contact effective from the computer systems to the computer peripheral, with the help of which the respective computer peripheral is informed that a data information is ready for a transmission thereto.
a bidirectionally controllable "reset" interface contact, which indicates the reset condition, i.e. a defined operation output condition either to a computer peripheral or to a computer system. An installation is placed in its reset condition after disturbances, failures or given an initial operation, in order to set a defined output operational condition.
The interface contacts of the respective computer peripheral or computer systems which do not participate in an actual communication transmission are controlled in impedance fashion by the interface controls of the respective computer peripheral o the computer systems. By that means, the fault liability at the device interface is decreased and the energy consumption reduced. At the asymmetrical device interface, the computer peripheral or computer systems can be connected via maximally 6 meters long flat cables twisted in pairs.
During the operation of the computer systems no further computer peripheral or computer systems can be connected to the device interface since the capacitance and inductance changes caused by the connection can cause signal disturbances at the device interface and thus data transmission disturbances which can lead to a disturbance or to a shorttime failure of the computer systems.